1. Field of the Invention
The present invention relates to the field of power plant technology. It pertains to a method for operating a (stationary) gas turbine according, as well as a gas turbine for implementing the method.
2. Brief Description of the Related Art
A gas turbine with reheating (reheat gas turbine) is known (see, for example, the U.S. Pat. No. 5,577,378 or “State-of-the-art gas turbines—a brief update,” ABB Review 02/1997, FIG. 15, turbine type GT26), which combines flexible operation with very low flue gas emission values.
The machinery architecture of the gas turbine of Type GT26 is unique and is exceptionally well-suited to realizing a concept that is the subject matter of the present invention, because:                even in the case of the compressor, there is a significant diversion of compressor air at intermediate compressor pressures,        the concept of sequential combustion renders an increased stability of combustion possible in conjunction with reduced levels of excess oxygen, and        a secondary air system is present, which renders it possible to divert air from the compressor, to cool it down, and to use the cooled air for cooling the combustor and the turbine.        
The principle of the known gas turbine with reheating is shown in FIG. 1. The gas turbine 11, which is a portion of a combined cycle power plant 10, includes two connected compressors, arranged behind one another on a commonly shared shaft 15, namely a low pressure compressor 13 and a high pressure compressor 14, as well as two combustors, namely a high pressure combustor 18 and a reheat combustor 19, and the pertinent turbines, namely a high pressure turbine 16 and a low pressure turbine 17. The shaft 15 drives a generator 12.
The manner in which the unit works is as follows: air is drawn in via an air inlet 20 from the low pressure compressor 13, and is compressed initially to a level of intermediate pressure (ca. 20 bar). The high pressure compressor 14 then further compresses the air to a level of high pressure (ca. 32 bar). Cooling air is diverted at both the level of intermediate pressure and at the level of high pressure and cooled down in pertinent OTC coolers (OTC=Once Through Cooler) 23 and 24 and conducted further to the combustors 18 and 19 and turbines 16, 17 via cooling lines 25 and 26 for cooling purposes. The remaining air from the high pressure compressor 14 is conducted to the high pressure combustor 18 and heated there by the burning of a fuel, which is introduced via the fuel feedline 21. The resultant flue gas is then expanded in the downstream high pressure turbine 16 to an intermediate level of pressure as it performs work. After expansion, the flue gas is reheated in the reheat combustor 19 by the burning of a fuel that is introduced via fuel feedline 22 before it is expanded in the downstream low pressure turbine 17, performing additional work in the process.
The cooling air, which flows through the cooling lines 25, 26, is sprayed in at suitable points of the combustors 18, 19 and turbines 16, 17 to limit material temperatures to a reasonable degree. The flue gas, which comes from the low pressure turbine 17, is sent through a heat recovery steam generator 27 (HRSG) in order to generate steam, which flows within a water-steam circuit through a steam turbine 29 and performs additional work there. After flowing through the heat recovery steam generator 27, the flue gas is finally released to the outside through a flue gas line 28. The OTC coolers 23, 24 are a portion of the water-steam circuit; superheated steam is generated at their outlets.
As a result of the two combustions in the combustors 18 and 19, which are independent of each other and follow one another, great flexibility of operation is achieved; the combustor temperatures can be adjusted in such a way that the maximum degree of efficiency is achieved within the existing limits. The low flue gas levels of the sequential combustion system are provided by the inherently low emission levels, which can be achieved in the course of reheating (under certain conditions, the second combustion even leads to a consumption of NOx).
On the other hand, combined cycle power plants with single stage combustion in the gas turbines are known (see, for example, U.S. Pat. Nos. A4,785,622 or B26,513,317), in which a coal gasification unit is integrated in order to provide the requisite fuel for the gas turbine in the form of syngas, which is recovered from coal. Such combined cycle power plants are designated IGCC (Integrated Gasification Combined Cycle) plants.
The present invention now proceeds from the recognition that due to the use of gas turbines with reheating in an IGCC plant, the advantages of this type of gas turbine can be made usable for the plant in a particular manner.